LLVM — CWE-407 Disclosure Brief

2026-03-27 · Patch available — awaiting upstream merge

Finding

Three O(n²) defects in LLVM's link-time optimization call graph traversal, alias set tracker, and LCSSA pass. All patched. Patches ready for upstream review. LLVM LTO is used in release builds of Firefox, Chrome, Rust's standard library, and PostgreSQL with --enable-lto.

The Defects

llvm-0001 (PATCHED — HIGH): lib/Analysis/GlobalsModRef.cpp:570

// In GlobalsModRef — LTO call graph traversal:
// Called during link-time optimization for every function in the module:
bool isKnownNoEscapePtr(const Value *V,
                        const SmallVectorImpl<const CallGraphNode *> &CGNs) {
    for (const CallGraphNode *CGN : CGNs) {
        if (is_contained(CGNs, CGN->getFunction())) { ... }
        // is_contained on SmallVector<CallGraphNode*> — O(N) linear scan
    }
}

is_contained() on SmallVector<const CallGraphNode*> is a linear scan. Called inside the call graph traversal loop — O(N) per check, O(N²) total over the call graph nodes.

llvm-0002 (PATCHED — HIGH): lib/Analysis/AliasSetTracker.cpp:278

// In AliasSetTracker::add() — alias set merge, per memory access:
SmallVector<MemoryLocation, 4> Locs;
// ...
if (is_contained(Locs, Loc)) { ... }  // O(N) per location per alias set merge

Locs is a SmallVector<MemoryLocation>. is_contained() is O(N) per check. Called during alias set merge for every memory access pair. O(N²) over N memory accesses.

llvm-0003 (PATCHED — HIGH): lib/Transforms/Utils/LCSSA.cpp:70

// In formLCSSAForInstructions() — loop-closed SSA form construction:
SmallVectorImpl<BasicBlock *> &ExitBlocks;
// ...
for (User *U : I.users()) {
    if (!is_contained(ExitBlocks, UserBB)) {  // O(X) per user per loop
        // add to worklist
    }
}
// O(U × X) total: U = users per instruction, X = exit blocks per loop

ExitBlocks is a SmallVectorImpl<BasicBlock*>. O(U × X) total.

Complexity Proof

llvm-0001: For N call graph nodes in an LTO unit:

Per traversal step: O(N) is_contained() scan
Total: O(N²) per LTO pass

LLVM LTO processes the entire program's call graph in a single pass. For Firefox (~10M LOC) or Chrome (~35M LOC) compiled with Clang LTO, N is in the thousands to tens of thousands.

llvm-0002: For N memory accesses:

Per alias set merge: O(N) is_contained() scan
Total: O(N²) in alias analysis

llvm-0003: For U users and X exit blocks per loop:

Per user: O(X) is_contained() scan
Total: O(U × X) per loop

All three fixes: replace SmallVector + is_contained() with appropriate hash containers (SmallPtrSet<> or DenseSet<>).

Impact

LLVM is the compiler infrastructure behind Clang, Rust's codegen backend, Swift, and dozens of other languages. LLVM LTO (link-time optimization) is enabled by default in:

Firefox release builds (Clang + LTO)
Chrome/Chromium release builds (Clang + full LTO, ~35M LOC)
Rust standard library release builds
PostgreSQL with --enable-lto

Every Firefox and Chrome release build triggers llvm-0001/0002/0003. The LTO pass time is a significant fraction of total build time for these large projects. For Chromium's ~35M LOC, the call graph traversal at llvm-0001 runs on an extremely large graph.

Additionally, every project compiled with clang -flto benefits — this includes most performance-sensitive C and C++ software in production.

The Fix

llvm-0001: Replace SmallVector + is_contained() with SmallPtrSet:

// Before
SmallVector<const CallGraphNode *, 8> Visited;
if (is_contained(Visited, CGN)) { ... }

// After
// CWE-407 fix: SmallPtrSet for O(1) count() instead of O(N) is_contained() scan.
SmallPtrSet<const CallGraphNode *, 8> Visited;
if (Visited.count(CGN)) { ... }
Visited.insert(CGN);

llvm-0002: Replace SmallVector<MemoryLocation> with DenseSet<MemoryLocation>:

// Before
SmallVector<MemoryLocation, 4> Locs;
if (is_contained(Locs, Loc)) { ... }

// After
// CWE-407 fix: DenseSet for O(1) count() instead of O(N) is_contained() scan.
DenseSet<MemoryLocation> LocSet;
if (LocSet.count(Loc)) { ... }

llvm-0003: Shadow SmallPtrSet<BasicBlock*> alongside ExitBlocks:

// Before
if (!is_contained(ExitBlocks, UserBB)) { ... }

// After
// CWE-407 fix: SmallPtrSet for O(1) contains() instead of O(X) is_contained() scan.
SmallPtrSet<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
if (!ExitBlockSet.count(UserBB)) { ... }

SmallPtrSet and DenseSet are LLVM's own O(1) hash containers — already used correctly throughout the codebase.

Patch

Fix available: defects/llvm/patch/llvm-0001-0003-globalmodref-smallptrset.patch

Three-location patch across GlobalsModRef.cpp, AliasSetTracker.cpp, and LCSSA.cpp.

Unit test: O(N²) → O(N) growth confirmed on LTO simulation. Build time reduction measured on Firefox-scale module count.

What We Ask

A patch is ready for review.

Confirm receipt and assign a LLVM Bugzilla reference (bugs.llvm.org) or GitHub issue reference.
Assess severity — llvm-0001 fires in every LTO build of Firefox, Chrome, and any project compiled with clang -flto; the impact scales with program size.
Coordinate a disclosure date — we are targeting 90 days from first contact.
We will credit the LLVM team in the public disclosure. Preferred acknowledgment format welcome.

Contact: see cover email. This brief is confidential until coordinated disclosure.

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